Article

Differential effects of SNAP-25 deletion on Ca2+-dependent and Ca2+-independent neurotransmission

Department of Neuroscience, The University of Texas Southwestern Medical Center, Dallas, Texas 75390-9111, USA.
Journal of Neurophysiology (Impact Factor: 3.04). 09/2007; 98(2):794-806. DOI: 10.1152/jn.00226.2007
Source: PubMed

ABSTRACT At the synapse, SNAP-25, along with syntaxin/HPC-1 and synaptobrevin/VAMP, forms SNARE N-ethylmaleimide-sensitive factor [soluble (NSF) attachment protein receptor] complexes that are thought to catalyze membrane fusion. Results from neuronal cultures of synaptobrevin-2 knockout (KO) mice showed that loss of synaptobrevin has a more severe effect on calcium-evoked release than on spontaneous release or on release evoked by hypertonicity. In this study, we recorded neurotransmitter release from neuronal cultures of SNAP-25 KO mice to determine whether they share this property. In neurons lacking SNAP-25, as those deficient in synaptobrevin-2, we found that approximately 10-12% of calcium-independent excitatory and inhibitory neurotransmitter release persisted. However, in contrast to synaptobrevin-2 knockouts, this remaining readily releasable pool in SNAP-25-deficient synapses was virtually insensitive to calcium-dependent-evoked stimulation. Although field stimulation reliably evoked neurotransmitter release in synaptobrevin-2 KO neurons, responses were rare in neurons lacking SNAP-25, and unlike synaptobrevin-2-deficient synapses, SNAP-25-deficient synapses did not exhibit facilitation of release during high-frequency stimulation. This severe loss of evoked exocytosis was matched by a reduction, but not a complete loss, of endocytosis during evoked stimulation. Moreover, synaptic vesicle turnover probed by FM-dye uptake and release during hypertonic stimulation was relatively unaffected by the absence of SNAP-25. This last difference indicates that in contrast to synaptobrevin, SNAP-25 does not directly function in endocytosis. Together, these results suggest that SNAP-25 has a more significant role in calcium-secretion coupling than synaptobrevin-2.

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    • "However, even a slight increase in VAMP proteins levels could accommodate successful vesicle fusion (van den Bogaart et al., 2010). Loss of function studies of key SNAREs involved in fusion in neuronal system suggest that a parallel pathway, possibly involving noncanonical SNAREs typically implicated in constitutive vesicle trafficking, may mediate fusion and recycling of a subset of vesicles (Bronk et al., 2007; Deak et al., 2004; Schoch et al., 2001; Washbourne et al., 2002; Ramirez and Kavalali, 2012). Certainly, there was no clear transcriptional activation of any of the SNARE genes that we examined in the knockdowns . "
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    • "For example, loss of SNAP25 or synaptobrevin largely abolishes calcium-dependent evoked release but leaves spontaneous fusion release intact suggesting a role for alternate SNAREs in mediating spontaneous release (Schoch et al., 2001; Washbourne et al., 2002; Bronk et al., 2007). The CLASP2-mediated increase in synapsin synaptobrevin, and SNAP25 may provide possible mechanistic detail of the functional increase in evoked mEPSCs as alterations in expression levels of these proteins have been shown to cause calcium-dependent evoked neurotransmitter release (Lu et al., 1992; Rosahl et al., 1995; Schoch et al., 2001; Washbourne et al., 2002; Bronk et al., 2007). "
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    • "or in inhibitory (mIPSCs) and excitatory (mEPSCs) spontaneous release events (Supplementary Figure S2D–I). Thus, a loss of the vast majority of SNAP-25 from neurons does not cause a major synaptic phenotype, as opposed to a complete loss of SNAP-25 in KO mice that blocks synaptic transmission (Washbourne et al, 2002; Bronk et al, 2007). This surprising result suggests that the small amount of reserve SNAP-25 protein remaining in KD neurons is largely sufficient for full presynaptic function. "
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